Coated substrate drying system with magnetic particle orientation

ABSTRACT

Drying a coated substrate which includes magnetic particles includes a condensing surface spaced from the substrate. This creates a longitudinal gap between the substrate and the condensing surface. Liquid is evaporated from the substrate to create a vapor and the vapor is transported to the condensing surface without requiring applied convection. The vapor is condensed on the condensing surface to create a condensate which is removed from the condensing surface. Removal is performed, using more than gravity, without allowing non-uniformities of the condensate film to occur. The magnetic particles are oriented on the coated substrate by subjecting the coated substrate to a magnetic field created at a location outside of the dryer space.

This is a continuation-in-part to U.S. application Ser. No. 08/699,522,filed Sept. 4, 1996, U.S. Pat. No. 5,694,701.

TECHNICAL FIELD

The present invention relates to a method and apparatus for transportingmass and energy and for drying coatings on a substrate. Moreparticularly, the present invention relates to substrate drying withmagnetic particle orientation.

BACKGROUND OF THE INVENTION

Drying coated substrates, such as webs, requires supplying energy to thecoating and then removing the evaporated liquid. The liquid to beevaporated from the coating can be any liquid including solvents such asorganic solvent systems and inorganic systems which include water-basedsolvent systems. Convection, conduction, radiation, and microwave energyare used to supply energy to coated webs. Applied convection or forcedgas flow is used to remove the evaporated liquid. Applied convection isdefined as convection produced by the input of power and causedintentionally. It excludes convection caused merely by web movement,natural convection, and other, unavoidable, forces. In some instanceswhere the vapors are non-toxic, such as water evaporation, the vapor isremoved by flashing off into the ambient atmosphere.

In conventional drying technology, large volumes of gas, inert or not,are required to remove evaporated liquid from the gas/liquid interface.These dryers require large spaces between the coated web being dried andthe top of the drying enclosure to accommodate the large gas flows.Drying is governed at the gas/liquid interface by diffusion, convection,boundary layer air from the moving web and impinging air streams, vaporconcentrations, and liquid to vapor change-of-state convection, amongother factors. These phenomena occur immediately above the coated web,typically within 15 cm of the surface. Because conventional dryers havea large space above the coated web, and they can only control theaverage velocity and temperature of the bulk gas stream, they havelimited ability to control these phenomena near the gas/liquidinterface.

For organic solvent systems, the vapor concentrations in these bulk gasstreams are kept low, typically 1-2%, to remain below the flammablelimits for the vapor/gas mixture. These large gas flows are intended toremove the evaporated liquid from the process. The expense to enclose,heat, pressurize, and control these gas flows is a major part of thedryer cost. It would be advantageous to eliminate the need for theselarge gas flows.

These gas streams can be directed to condensation systems to separatethe vapors before exhausting, using a large heat exchangers or chilledrolls with wiping blades. These condensation systems are locatedrelatively far from the coated web in the bulk gas flow stream. Due tothe low vapor concentration in this gas stream, these systems are large,expensive, and must operate at low temperatures.

It would be advantageous to locate the condensation systems close to thecoated substrate where the vapor concentrations are high. However,conventional heat exchangers would drain the condensed liquid by gravityback onto the coating surface and affect product quality unless theywere tilted or had a collection pan. If they had a collection pan theywould be isolated from the high concentration web surface. If they weretilted dripping would probably still be a problem. Also, conventionalheat exchangers are not planar to follow the web path and control thedrying conditions.

U.S. Pat. No. 4,365,423 describes a drying system which uses aforaminous surface above the web being dried to shield the coating fromturbulence produced by the large gas flows to prevent mottle. However,this system does not eliminate applied convection, requires usingsecondary, low efficiency solvent recovery, and has reduced dryingrates. Also, because of the reduced drying rates, this patent teachesusing this shield for only 5-25% of the dryer length.

German Offenlegungeschrift No. 4009797 describes a solvent recoverysystem located within a drying enclosure to remove evaporated liquid. Achilled roll with a scraping blade is placed above the web surface andremoves the vapors in liquid form. No applied convection removes theevaporated liquid. However, the roll is only in the high vaporconcentration near the surface for a short section of the dryer length.This does not provide optimal control of the conditions at thegas/liquid interface. In fact as the roll rotates it can createturbulence near the web surface. Also, this system can not adapt itsshape to the series of planar surfaces of the coated web as it travelsthrough the dryer. Therefore, the system can not operate with a small,planar gap to control drying conditions and can not achieve optimumcondensing efficiency.

U.K. patent No. 1 401 041 describes a solvent recovery system thatoperates without the large gas flows required for conventional drying byusing heating and condensing plates near the coated substrate. Thesolvent condenses on the condensing plate and then the condensed liquiddrains by gravity to a collection device. This apparatus uses onlygravity to remove the liquid from the condensing surface. Accordingly,the condensing surface can not be located above the coated substratesince gravity will carry the condensed liquid back onto the coatedsubstrate. In the drawings and discussion (page 3, lines 89-92) thecondensing surface is described as vertical or with the coatedsubstrate, coated side facing down, above the condensing surface.Applying a coating to the bottom side of the substrate or inverting thesubstrate after application of the coating is not the preferred methodin industry. Coating in an inverted position and inverting a coatedsubstrate before drying can create coating defects. These limitationsgreatly reduce the flexibility of the method and entail significantcosts to adapt it to standard manufacturing methods. This requirementfor vertical or inverted drying is very likely the reason this methodhas not been adopted or discussed in the industry.

U.K. patent No. 1 401 041 also describes, on page 2 line 126 to page 3line 20, the problems of this method with growth of the liquid filmlayer on the condensing surface and droplet formation. Because "theresulting liquid film 14 may increase in thickness towards the lower endof the condenser," the length of the condensing surface is limited bythe buildup and stability of this film layer. Limiting the length of thecondensing surface will limit the dryer length or require exiting thedrying system with the coating not dried. This has the undesirableeffect of losing some of the solvent vapors to the atmosphere, losingcontrol of the drying phenomena, and creating defects. Anotherlimitation is that the distance of the condensing surface from thecoated substrate "can hardly fall below about 5 millimeters" to preventcontacting the condensing liquid film with the substrate, and to preventdroplets from contacting the substrate.

The limitations of this system to vertical or inverted drying, limits inthe length of the dryer, and the inability to operate at desireddistances from the coated substrate render it inadequate to achieve thedesired drying benefits.

Dryers used to dry magnetic coatings also are known. Known systems placea magnetic field generator inside the dryer to orient the magneticparticles within the coating being dried. However, conventionalorienting devices inside the dryer disrupt the air flow and impairdrying, causing the surface of the product to roughen. As the particlesleave a conventional orienting device in the early stages of drying, anycomponents of the magnetic field which are not in the plane of thecoating will reorient the particles in a non-preferred direction.

There is a need for a system for drying coated substrates whileorienting magnetic particles in the coating, which provides improvedcontrol of the conditions near the gas/liquid interface and in which theorientation process does not interfere with the drying. There is also aneed for a system that can operate with small gaps adjacent thesubstrate.

SUMMARY OF THE INVENTION

The invention is a method and apparatus of drying a coated substratethat includes magnetic particles. A condensing surface is spaced fromthe substrate and substantially corresponds to the path of the substratein the longitudinal direction. This creates a longitudinal gap betweenthe substrate and the condensing surface. Liquid is evaporated from thesubstrate to create a vapor and the vapor is transported to thecondensing surface without requiring applied convection. The vapor iscondensed on the condensing surface to create a condensate, and thecondensate is removed from the condensing surface. Removal is performed,using more than gravity, without allowing non-uniformities of thecondensate film to occur. The magnetic particles are oriented on thecoated substrate by subjecting the coated substrate to a magnetic fieldcreated at a location outside of the space between the condensingsurface and the coated substrate. The magnetic field can be created by amagnetic field generator.

The magnetic field can be created at various locations: a locationseparated from the substrate by the condensing surface, a locationseparated from the condensing surface by the substrate, a locationsurrounding the substrate, and any combination of these locations.

The magnetic particles can be oriented at the beginning of theevaporating step and holding the magnetic particles in a preferreddirection during the evaporating, condensing, transporting, and removingsteps.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the drying and orienting apparatus ofthe invention.

FIG. 2 is an end view of the apparatus of FIG. 1.

FIG. 3 is a cross-sectional view taken along line 3--3 of FIG. 1.

FIG. 4 is a perspective view of the drying and orienting apparatusaccording to another embodiment of the invention.

FIG. 5 is an end view of the apparatus of FIG. 4.

FIG. 6 is a perspective view of another embodiment of the dryingapparatus.

FIG. 7 is a cross-sectional view of another embodiment of the dryingapparatus.

FIG. 8 is a cross-sectional view of another embodiment of the dryingapparatus.

FIG. 9 is a cross-sectional view of another embodiment of the dryingapparatus.

FIG. 10 is a schematic side view of another embodiment of the dryingapparatus.

FIG. 11 is a cross-sectional view of another embodiment of the dryingapparatus.

FIG. 12 is a bottom view of a condensing platen according to anotherembodiment of the invention.

FIG. 13 is a top view of another embodiment of the drying apparatus.

FIG. 14 is a top view of the drying apparatus according to anotherembodiment of the invention.

FIG. 15 is a side view of another embodiment of the drying apparatus.

FIG. 16 is a schematic side view of the invention showing processvariables.

DETAILED DESCRIPTION

The system of this invention is a method and apparatus for transportingmass and energy and for drying coatings on a coated substrate, such as amoving web, with a condensing surface creating a small,controlled-environment gap above the coating surface. Other physical andchemical phenomena that occur during the drying process, such aschemical reactions, curing, and phase changes, can also be affected bythe invention. The coated substrate includes magnetic particles,particles which are capable of being attracted or repelled by a magnet,such as ferrous particles used in audio, video, and data storage tape ormedia. The method and apparatus also orient these particles in amagnetic field as described below.

In the embodiment of FIGS. 1, 2, and 3, drying (heating the liquid toevaporate it to a vapor, transporting the vapor away from the web,condensing the vapor, and transporting the condensed vapor (also knownas condensate) away from the web) occurs without requiring the appliedgas convection associated with conventional drying methods. This reducesmottle formation associated with many precision coatings and enablesdrying at increased drying rates. In the embodiment of FIGS. 4-15, atleast the removal of the evaporated liquid from the web occurs withoutrequiring applied gas convection. All versions of this system attainimproved control of the phenomena occurring near the gas/liquidinterface and attain high liquid recovery efficiencies.

All versions use condensation to remove evaporated liquid in a gap whichcan be substantially planar without requiring applied convection forces,and where ambient and boundary layer convection forces are minimized.The drying system has numerous advantages over the conventional dryingtechnology by creating a small, controlled-environment gap adjacent thecoating surface, and by eliminating the requirement for appliedconvection from the drying mechanism. In some products a chemicalreaction or other physical and chemical processes occur in the coatingduring drying. The drying system functions whether or not theseprocesses are proceeding within the process. The drying system canaffect these processes during drying. One example is of moisture-curedpolymers dispersed or dissolved in a solvent that can be adverselyaffected during the drying process due to the presence of humidity inthe drying atmosphere. Because the invention can create a small,controlled environment gap above the coating surface, it issubstantially simpler to provide a controlled humidity drying atmosphereto improve the curing of these polymers. By improving control of thedrying phenomena and creating a small, controlled environment gap abovethe coated surface, there are many other applications where otherphysical and chemical processes occurring during the drying process canbenefit.

In an alternative method the drying system can be combined with appliedconvection, and the applied convection can be produced by forcing gasacross the coating, either longitudinally, transversely, or in any otherdirection. This can provide additional mass transfer or othermodification to the atmosphere above the coated surface. This methodcould be used where applied convection is not a detriment to productproperties.

The inventors have found that in drying coated substrates, significantdrying improvements and increased drying rates occur when the distancefrom the condensing surface to the coated substrate is below 5millimeters. The system of U.K. patent No. 1 401 041 is not practicallyoperable in the range where significant drying control improvements canbe made.

Many kinds of condensing structures can be used, such as plates of anytype, whether flat or not, porous or not, structured or not, or othershapes such as tubes or fins. The condensing surface structure cancombine macro, meso, and micro scale geometries and dimensions. Platesinclude fixed or moving platens, moving belts with or without liquidscrapers, and similar devices. The condensing structure can be parallelto the web or angled with the web, and can have planar or curvedsurfaces.

The condensing surface must satisfy three criteria. First, it must becapable of sufficient energy transfer to remove the latent heat ofcondensation. Second, the condensate must at least partially wet thecondensing surface. Third, the condensing surface must prevent thecondensed vapor (the condensate) from returning to the coated surface ofthe web. Associated with a condensing surface is an effective criticalcondensate film thickness which marks the onset of filmnon-uniformities. This thickness is a function of the condensing surfacematerial, geometry, dimensions, topology, orientation, configuration,and other factors, as well as the physical properties of the condensate(such as surface tension, density, and viscosity). Another feature ofthe system is condensate transport and removal. This maintains thecondensate film thickness less than the effective critical thickness andcan be accomplished by capillary forces, gravitational forces,mechanical forces, or various combinations of these forces.

Capillary force, or capillary pressure, can be described as theresultant of surface tension acting in curved menisci and is governed bythe fundamental equation of capillarity known as the Young-LaPlaceequation. The Young-LaPlace equation is ΔP=σ(1/R₁ +1/R₂), where ΔP isthe pressure drop across the interface, σ is the surface tension, and R₁and R₂ are the principal radii of curvature of the interface.Capillarity is discussed in detail in Adamson, A. W. "Physical Chemistryof Surfaces, 4th ed.", John Wiley & Sons, Inc. (1982). FIGS. 1, 2, 4, 5,9, 10, and 11 show examples of using capillary forces, along with otherforces, to remove the condensate from the condensing surface.

Gravitational forces result from the position of the fluid mass in agravitational field, which is the hydrostatic head. FIGS. 7, 8, 10, and12 show examples that use gravitational forces, along with other forces,to remove the condensate from the condensing surface.

Other mechanisms can be used to remove the condensed liquid from thecondensing surface to prevent the condensed liquid from returning to thesubstrate. For example, mechanical devices, such as wipers, belts,scrapers, pumping systems, or any combination, can be used to remove thecondensed liquid. FIGS. 6, 13, 14, and 15 show examples that usemechanical forces, along with other forces, to remove the condensatefrom the condensing surface.

FIGS. 1, 2, and 3 show an apparatus using two platens. FIGS. 4 and 5show an apparatus using one platen. In both versions, one platen has acondensing, liquid-transport surface located a short distance from thecoated surface of the web. Distances of less than 15-20 cm arepreferred. Distances less than 5 mm yield more advantages. Distancesless than 0.5 mm and even distances as low as 0.1 mm and less areattainable.

In FIGS. 1 and 2, the apparatus 10 includes a condensing platen 12,which can be chilled, spaced from a heated platen 14. A solenoid coil 15surrounds the platens 12, 14. The condensing platen 12 is set to atemperature T₁, which can be above or below ambient temperature, and theheated platen 14 is set to a temperature T₂, which can be above or belowambient temperature. The coated web 16 temperature is T₃. The webposition is defined by h₁ and h₂, the distances between the respectivefacing surfaces of the web 16 and the condensing and heated platens.FIG. 16 shows the relative locations of these variables. The total gapbetween the condensing platen and any heating platen, h, is the total ofh₁, h₂, and the coated web thickness. The web 16, having a coating 18,travels at any speed between the two platens. Alternatively, the web canbe stationary and the entire apparatus 10 moves or both the web andapparatus move. The platens are stationary within the apparatus. Theheated platen 14 is located on the non-coated side of the web 16, eitherin contact with the web or with a small gap h₂ between the web and theplaten. The condensing platen 12 is located on the coated side of theweb 16, with a small gap h₁ between the web and the platen. Thecondensing platen 12 and the heated platen 14 eliminate the requirementfor applied convection forces both above and below the web 16. Drying iscontrolled by adjusting the temperatures T₁, T₂, and distances h₁, h₂.

The condensing platen 12, which can be stationary or mobile, is placednear the coated surface (such as 10 cm away, 5 cm away, or closer). Thearrangement of the platens creates a small gap adjacent the coated web.The gap is substantially constant, which permits small amounts ofconvergence or divergence. Also, the gap is substantially constantnotwithstanding any grooves (discussed below) on the condensing surface.The orientation of the platens is not critical. The condensing platen 12can be above the web (as shown in FIGS. 1, 2, 4, and 5-9), below the web(with the coating on the bottom surface of the web), and the system canoperate with the web vertical or at any other angle, including beingtilted around the axis of the direction of web travel.

The heated platen 14 supplies energy without requiring appliedconvection through the web 16 to the coating 18 to evaporate liquid fromthe coating 18 to dry the coating. Energy is transferred by acombination of conduction, radiation, and convection achieving high heattransfer rates. This evaporates the liquid in the coating 18 on the web16. The evaporated liquid from the coating 18 then is transported (usingdiffusion and convection) across the gap h₁ between the web 16 and thecondensing platen 12 and condenses on the bottom surface of thecondensing platen 12.

As shown in FIG. 3, the bottom surface of the condensing platen 12 isthe condensing surface 22 and has transverse open channels or grooves 24which use capillary forces to prevent the condensed liquid fromreturning to the coating by gravity and to move the condensed liquidlaterally to edge plates 26. The grooves can be triangular, rectangular,circular, or other more complex shapes or combinations of shapes. Thegroove material, geometry, and dimensions are designed to accommodatethe required mass flow and the physical properties of the condensate,such as surface tension, viscosity, and density.

A specific type of condensing surface is one which has open channels orgrooves with corners. This type of capillary condensing surface, shownfor example in FIG. 3, is a geometrically specific surface which can bedesigned with the aid of the Concus-Finn Inequality (Concus P. and FinnR. "On the Behavior of a Capillary Surface in a Wedge," Proceeding ofthe National Academy of Science, vol. 63, 292-299 (1969)) which is:α+θ_(s) <90°, where a is half the included angle of any corner and θ_(s)is the gas/liquid/solid static contact angle. The static contact angleis governed by the surface tension of the liquid for a given surfacematerial in gas. If the inequality is not satisfied, the interface isbounded; if the inequality is satisfied, the interface does not have afinite equilibrium position and the meniscus is unbounded. In thislatter case, the liquid will advance by capillarity indefinitely or tothe end of the channel or groove. Cornered grooved surfaces are helpfulwhen the coating liquid has a high surface tension, such as water.Capillary surfaces with corners are discussed in great detail in Lopezde Ramos, A. L., "Capillary Enhanced Diffusion of CO₂ in Porous Media,"Ph.D. Dissertation, University of Tulsa (1993).

The grooves 24 also can be longitudinal or in any other direction. Ifthe grooves are in the longitudinal direction, a suitable collectionsystem can be placed at the ends of the grooves to prevent the condensedliquid from falling back to the coated surface 18. This embodimentlimits the length of a condensing plate 12 and also limits the minimumgap h₁.

When the liquid reaches the end of the grooves 24 it intersects with theangle between the edge plates 26 and the condensing surface 22. A liquidmeniscus forms and creates a low pressure region which draws thecondensate from the condensing surface to at least one edge plate.Gravity overcomes the capillary force in the meniscus and the liquidflows as a film or droplets 28 down the face of the edge plates 26. Theedge plates 26 can be used with any condensing surface, not just onehaving grooves. The droplets 28 fall from each edge plate 26 and can becollected in a collecting device (not shown). For example, a slottedpipe can be placed around the bottom edge of each edge plate 26 tocollect the liquid and direct it to a container. The edge plates 26 areshown throughout the application as contacting the ends of thecondensing surface of the condensing platens. However, the edge platescan be adjacent the condensing platens without contacting them as longas they are functionally close enough to receive the condensed liquid.

Alternatively, the condensed liquid need not be removed from the platenat all, as long as it is removed from the condensing surface 22, or atleast prevented from returning to the web 16. Also, the edge plates 26are shown as perpendicular to the condensing surface 14, although theycan be at other angles with it, and the edge plates 26 can be smooth,grooved, porous, or other materials.

The heated platen 14 and the condensing platen 12 can include internalpassageways, such as channels. A heat transfer fluid can be heated by anexternal heating system and circulated through the passageways to setthe temperature T₂ of the heated platen 14. The same or a different heattransfer fluid can be cooled by an external chiller and circulatedthrough the passageways to set the temperature T₁ of the condensingplaten 12. Other mechanisms for heating the platen 14 and cooling theplaten 12 can be used.

The apparatus 30 of FIGS. 4 and 5 is similar to that of FIGS. 1-3 exceptthere is no heating platen. In the apparatus 30, the web 16 is heated toevaporate the liquid from the coating by any heating method orcombination of heating methods, whether conduction, radiation,microwave, convection, or ambient energy, using any type of heater. Thiscan include but is not limited to a heated drum, radiant heatingdevices, or forced gas flows. This system can even operate without anyapplied energy, even outside the dryer, using only ambient energy toevaporate the liquid. The apparatus 30 otherwise operates the same asthat of FIGS. 1-3, without requiring applied convection for transport ofthe evaporated liquid from the web 16 to the condensing surface 22 onthe condensing platen 12. The gap h₁ between the coated web 16 and thecondensing surface 22 is isolated from the heating devices by anycombination of the web 16 and web supports or other barriers. This canisolate the area from any applied convection.

In FIG. 6, the apparatus 32 includes a belt 34 which has the condensingsurface 22. The belt 34 substantially corresponds to the shape of thesubstrate and provides a gap between the substrate and the condensingsurface. The belt can be solid or porous and can be made of a variety ofmaterials. The belt is driven by rollers 36 which can provide relativemovement between the condensing surface 22 and the substrate 16.Alternatively the condensing surface 22 can be driven to provide nomovement relative to the web 16 or it can be driven in the oppositedirection of the web 16. Alternatively the entire system can be rotatedfrom the position shown and the belt 34 can be driven substantiallytransverse to the direction of movement of the web 16. In this methodthe liquid would be removed beyond the edge of the web 16. Removal ofthe liquid from the condensing surface 22 is provided by a mechanicalwipe 38 which is adjacent to the belt 34. The mechanical wipe 38 removesthe liquid from the condensing surface 22 using shear forces and directsit to a suitable collection device 40.

FIGS. 7 and 8 show embodiments of the apparatus where gravity is used toremove the liquid solvent from the condensing surface. The condensingsurface 22 is on a plate 42 which is tilted to one transverse side ofthe web 16 in FIG. 7 and the condensing surface 22 is on one or twoplates 44 which are tilted from the center to both transverse sides ofthe web 16 in FIG. 8. In both cases gravity is used to move the liquidaway from the condensing surface. The angle could be centered on thelongitudinal centerline of the web or it can be off-center. Capillaritycan be combined with gravity.

FIG. 9 is another embodiment where capillary forces remove the liquidfrom the condensing surface. In this embodiment the condensing plate 46is a porous or wicking material, such as sintered metal or sponge, whichuses capillary forces to transport the liquid solvent. The solventcondenses on the condensing surface 22 and is distributed throughout thecondensing plate 46 due to capillary forces. The edge plates 26 adjacentthe condensing plate 46 form a capillary surface. A liquid meniscusforms and creates a low pressure region which draws the condensate fromthe condensing surface to at least one edge plate. Gravity overcomes thecapillary force and the liquid flows as a film or droplets down thesurface of the edge plate 26.

FIG. 10 shows another embodiment where capillary and gravity forces areused to transport the condensed liquid from the condensing surfaces 22.As shown, condensing surfaces 22 are formed on many surfaces. Acondensing platen 48 is tilted to one side or from the center to bothsides above the web 16. Thin sheets 50 of material are suspended belowthe condensing platen 48 and located such that they are slanted awayfrom the horizontal with their lower edge facing the lower edge of thecondensing platen 48. As shown, the sheets 50 of material overlap by atleast 0.05 cm and are spaced apart in the overlap region by a 0.01-0.25cm slot. Vapor that condenses on the condensing surfaces 22 will beretained on the surfaces by surface tension. Gravity carries thecondensed liquid down each upper surface of the sheets 50 in a cascadeeffect until the liquid is beyond the edge of the web 16. Liquid that iscondensed on the lower surface of the thin sheets 50 will transport tothe overlap region and capillary forces created by the slot will drawthe liquid into the slot. The liquid will then be transferred to theupper surface of the next sheet 50 and gravity will carry it in acascade manner to the edge of the substrate. Thus, liquid condensing onthe lower surface of the sheets will not form droplets that fall back tothe coated substrate. In some cases it is desirable for the liquid tocompletely fill the slot between the sheets 50 and the condensing platen48.

FIG. 11 is another embodiment which can combine gravity and capillaryforces to transport the liquid from the condensing surface. In thisembodiment a porous, slotted, sponge, honeycomb, screened, or otherwiseforaminous material 52 is attached to and located below a condensingplaten 54. The spacing between the condensing platen 54 and theforaminous material 52, the dimensions of the foramina in the material52, and the ratio of open area to solid area on the foraminous material52 are all designed to cause the surface tension forces to retain theliquid on the three condensing surfaces 22. The apparatus is locatedadjacent to the web 16. Vapor condensing on the condensing surfaces 22will be retained as liquid in the voids of the foraminous material andin the plate spacing region 56. As liquid is removed from the platespacing region 56, liquid on the side of the foraminous material 52facing the web 16 will be transported by capillary forces to fill thevoid in the plate spacing region 56. Liquid can be removed from theplate spacing region 56 either by gravity, capillary, or mechanicalforces. By sloping the condensing platen 54 away from the horizontal inany direction, gravitational forces will remove liquid from the platespacing region 56 to a point beyond the edge of the web 16.Alternatively, the liquid can be removed from the plate spacing region56 by positioning at least one edge plate 26 at the edge of thecondensing platen 54. The edge plate 26 contacts the condensing platen54 to form a capillary surface. The edge plates can, in some uses,contact the foraminous material 22. A liquid meniscus forms and createsa low pressure region which draws the condensate toward at least oneedge plate. Gravity overcomes the capillary force and the liquid flowsas a film or droplets down the surface of the edge plate 26. Also, thecondensate can be mechanically pumped out of the plate spacing region56.

FIG. 12 shows a condensing platen 60 with protruding structures. Thecondensing platen 60 provides a condensing surface 22 that cansubstantially correspond to the shape of the web 16. Gravity is used toremove the liquid from the condensing surface 22 by positioning theplaten 60 away from horizontal. This tilting from the horizontal can bein any direction, including transverse and parallel to the web 16 path.Without any additional device, the liquid draining from the condensingsurface 22 will, over a short distance (typically less than a meter),build a sufficient film thickness such that the surface tension forceswill be incapable of retaining the liquid and the liquid will fall asdroplets onto the web 16. Structure having any geometric shape, such asribs 62, can be positioned on the condensing surface 22 of thecondensing platen 60 to limit the buildup of film thickness, and preventthe formation of droplets that fall onto the web 16. The ribs 62 arelocated diagonally to the slope of the condensing surface 22 to directthe liquid beyond the edge of the web 16 to a suitable collecting device(not shown). They are provided in sufficient number and at a suitablespacing to limit the surface area drained by a specific rib 62 therebymaintaining the film thickness below the critical point for theoccurrence of droplet formation. The condensing surface can have groovesthat run in the longitudinal web direction.

The apparatus 64 of FIG. 13 mechanically moves the condensing surfaceand condensed liquid beyond the edge of the web 16 where the liquid isthen removed. A condensing platen 66 provides the condensing surface 22that is located adjacent to the web 16. The platen 66, which can becircular or any other shape, is mechanically rotated so that the liquidthat condenses on its condensing surface 22 is transported to an areabeyond the edge of the web 16. Removal of the liquid from the condensingsurface 22 is provided by a mechanical wipe 68 which is adjacent thecondensing surface 22 and anchored to a block 69. The mechanical wipe 68uses shear forces to remove the liquid from the condensing surface 22and direct it to a suitable collection device 70. A series of thesesystems can be located such that they substantially correspond to theshape of the substrate in the longitudinal direction.

FIG. 14 shows an apparatus 72 that uses surface tension to retain theliquid and a mechanical device to remove the liquid from the condensingsurface. A condensing platen 74 provides a condensing surface 22 thatcan substantially correspond to the shape of the web 16. Liquid thatcondenses on the condensing surface 22 is retained on that surface bysurface tension. Removal of the liquid from the condensing surface 22 isprovided by one or more mechanical wipes 76 which is adjacent thecondensing surface 22. The mechanical wipe 76 can move across thecondensing surface 22 transverse to the path of the web 16, parallel tothe path of the web 16, or in any other direction. The mechanical wipe76 uses shear forces to remove the liquid from the condensing surface 22and direct it to a suitable collection device 78 located below themechanical wipe 76. The liquid is carried in the collection device 78beyond the edge of the web 16 where it is transferred away.

FIG. 15 schematically shows an embodiment which uses a pump 80 to removethe condensed liquid from the condensing surface. The pump can be anytype of pump, and any other device for creating negative pressure can beused. As also shown in FIG. 15, the condensed liquid can be driventoward the transverse center of the condensing surface before removal,such as by capillarity and gravity.

In another use, the system can first remove fluid from a coatedsubstrate. Then, the system, at a downweb location from the dryinglocation, can be used "in reverse" to add some small portion of moistureor additional reactant to the substrate to modify the coating.

The apparatus can operate outside of a dryer configuration without anyapplied energy, and with only ambient heat to evaporate the liquid. Bycontrolling the temperature of the condensing surface 22 to be at ornear the ambient temperature, the liquid evaporation will only occuruntil the vapor concentration in the gap h₁ between the condensingsurface and the web 16 is at the saturated concentration as defined bythe condensing surface 22 and web 16 temperatures. The liquid that hasevaporated will be contained and carried by the viscous drag of the webthrough the gap h₁ to the exit of the system. Undesirable drying can bereduced and vapor emissions can be isolated from ambient conditions.

The drying system of the invention can be used to reduce or virtuallystop the drying of the coating. The rate of drying is a function of thegap height and vapor concentration gradient between the coated surface18 of the web 16 and the condensing surface 22. For a given gap h₁, thetemperature differential between the web 16 and the condensing surface22 defines the vapor concentration gradient. The higher the coatedsurface 18 temperature relative to the condensing surface 22, thegreater the rate of drying. As the temperature of the condensing surface22 approaches the coated surface 18 temperature, the drying rate willtend to zero. In conventional drying the vapor concentration gradientcannot be controlled without using an expensive inert gas drying system.Some liquid coatings have multiple solvents where one or more of thesolvents function to slow down the rate of drying for optimum productproperties. By adjusting the coated surface 18 and condensing surface 22temperatures, the invention can reduce the drying rate and possiblyeliminate the requirement of using solvents to retard the drying rate.

The rate of drying is controlled by the height of the gap h₁ and thetemperature differential between the coated surface 18 and thecondensing surface 22. Therefore for a given temperature differential,the rate of drying can be controlled by the position of the condensingplate which defines the gap h₁. Thus by changing the dimensions of thedrying system, such as by changing the relative gaps, it is possible tocontrol the rate of drying. Conventional dryers do not have thiscapability.

Drying some coated webs using applied convection can create mottlepatterns in the coatings. Mottle patterns are defects in film coatingsthat are formed by vapor concentration or gas velocity gradients abovethe coating which cause non-uniform drying at the liquid surface. Normalroom air currents are often sufficient to create these defects. Theinvention can be used to reduce and control natural convection induceddefects, such as mottle, at locations outside the desired dryingposition. In locations where the coated surface is not in the dryingregion and would otherwise be exposed to convection from either ambientair currents or from a turbulent boundary layer air due to web movement,the apparatus, with grooves or other liquid transport and removalfeatures, devices, structures or without, can be located adjacent to thecoated web 16 separated by a gap h₁. The location of the condensingplate 12 adjacent the coated web 16 can isolate the ambient air currentsfrom the coating surface. It can also prevent the boundary layer airabove the coated surface from becoming turbulent. Accordingly, defectsdue to convection outside the drying position, such as mottle, can bereduced or eliminated. The apparatus can be operated with condensationand solvent removal similar to FIGS. 4-15, or it can even operatewithout condensation and solvent removal by raising the condensingsurface 22 temperature above the dew point of the vapors in the gap h₁.

In all embodiments it may be desirable to provide multiple zones ofheating and condensing components using multiple pairs. The temperaturesand gaps of each pair of heating and condensing components can becontrolled independently of the other pairs. The zones can be spacedfrom each other or not.

The systems of all of the embodiments use condensation close to thecoated web 16 with a small gap between the coating on the web 16 and thecondensing surface 22. There is no requirement for applied convectionand there is very little vapor volume. The vapor concentration andconvection forces can be controlled by adjusting the web temperature,the gap, and the condensing surface temperature. This provides improvedcontrol of the conditions near the gas/liquid interface. Because theplate temperatures and gap can be continuous and constant throughout thedrying system, heat and mass transfer rates are more uniformlycontrolled than with conventional drying systems. All of these factorscontribute to improved drying performance. It also improves theefficiency of the condensation vapor recovery systems, providing forliquid recovery at high efficiencies at no additional cost compared toknown expensive methods of burning, adsorption, or condensation in asecondary gas stream.

Also, there is less of a concern about the ambient air above the webexploding or being above the flammability limit. In fact, where the gapis very small, such as less than 1 cm, flammability concerns may beeliminated because the entire space above the web has insufficientoxygen to support flammability. Additionally, this system eliminates theneed for large gas flows. The mechanical equipment and control system isonly 20% of the cost of a conventional air flotation drying system.

Experiments were conducted with 30.5 cm wide platens having transversegrooves. The bottom platen was heated to temperatures in the range of15° C. through 190° C. with a heat transfer fluid circulated throughpassageways in the platens. As the heat is transferred to the coating,the liquid in the coating evaporates. The temperature of the condensingplaten was controlled by any suitable method in the range of -10° C.through 65° C. to provide the driving force for vapor transport andcondensation. An effective range of the gap h₁ is 0.15-5 cm. Mottle-freecoatings were obtained.

In one example, a mottle-prone polymer/MEK solution at 11.5% solids, 2centipoise, 7.6 micron wet thickness, and 20.3 cm wide was coated. Theweb was 21.6 cm wide and traveled at a speed of 0.635 m/s. Thetemperature of the heated platen used to heat the web was controlled at82° C. The condensing platen temperature was controlled at 27° C. Theoverall length of the platens was 1.68 m and they were mounted at a 3.4°angle from horizontal with the inlet side at a lower elevation. Theinlet to the platens was located 76 cm from the coating applicationpoint. The heated platen was separated from the web by a gap ofapproximately 0.076 cm. The gap h₁ was set at 0.32 cm. The capillarygrooves were 0.0381 cm deep with a 0.076 cm peak-to-peak distance, anangle α of 30°, and 0.013 cm land at the top of the grooves. The web wasdried mottle-free in the 1.68 m length of the platens although there wassome residual solvent in the coating when it left the platens. Aconventional dryer would require approximately 9 m to reach the samedrying point, requiring the dryer to be more than five times larger.

Other applications for this system include drying adhesives whereblister defects are common. Blister defects may be caused by the coatingsurface forming a dried skin before the rest of the coating has dried,trapping solvent below this skin. With conventional drying, the solventvapor concentration in the bulk gas is very low because of flammabilitylimits. If too much heat is applied to the coating, the solvent at thesurface will flash very quickly into the low vapor concentration gasstream and will form the skin on the surface. The system of thisinvention creates a controlled vapor concentration in the space abovethe web which can reduce the tendency to form a skin on the surface.Other applications are in areas where dryers are run at high solventconcentrations to obtain specific product performance.

This system simplifies the process of subjecting the coating fluid to amagnetic field. Rather than positioning a magnetic field generatorwithin a known dryer, with the present invention the magnetic fieldgenerator can be positioned outside of the dryer (outside of apparatus10, 30). Locating the magnetic field generator outside of the dryermeans that it is outside of the space between the condensing surface andthe coated substrate. For example, it could be adjacent (whether spacedfrom or not) a side of the condensing plate opposite the side with thecondensing surface (location separated from the substrate by thecondensing surface); it could be adjacent a surface opposite the coatedsurface of the substrate (separated from the condensing surface by thesubstrate); or it could be at other locations. At these isolatedlocations, the magnetic field generator can use bucking fields to orientthe magnetic particles. Also, as described below, the magnetic fieldgenerator can surround the dryer.

The magnetic field generator can be any of various known devicesincluding permanent magnets and solenoid coils. A solenoid coil 15, asshown in FIG. 2, can have coil openings sufficiently large to permit thedryer to fit within the coils. This allows using the magnetic fieldinside of the coil openings which is uniform at all locations. Thisobviates any need to precisely locate the coated substrate within themagnetic field; variations in the substrate location will not affect thefield through which the substrate passes. The figures schematically showone coil 15, although a series of 2-10 or any other number of coils canbe used. These coils can be individually controlled and lower magneticfield strengths can be used.

This configuration is enabled by the compact nature of the apparatus.This is especially suitable when coating a metal particulate-loadedfluid onto a substrate to make such products as video and audiorecording tape, computer and data storage tape, computer diskettes, andthe like. Being outside of the apparatus, the magnetic field generatorsare easily adjustable and maintained.

This setup also improves magnetic output by physically orienting theparticles in the direction of recording. One advantage of this is thatthe magnetic orienting device is outside of the dryer and isnon-intrusive (conventional orienting devices inside the dryer disruptthe convection heat and mass transfer (air flow) and orient theparticles at a single point or multiple points as the solvent isremoved). Because orientation is non-intrusive, it will not affect thesolvent removal rates in any way. This allows uniform solvent removaland uniform coating drying while exposing the substrate to the magneticfield. The magnetic particles are easily oriented when the fluid is lessviscous at the early stages of drying with this invention. (As theparticles leave a conventional orienting device in the early stages ofdrying, any components of the magnetic field which are not in the planeof the coating will reorient the particles in a non-preferred direction,such as vertical. As the solvent is removed, the viscosity increases,making it difficult for the orienting device to rotate the particles.The particles will not be reoriented when leaving the field or byinterparticle forces.)

Another advantage is that because of its small size and increasedsolvent removal rates, the invention allows orienting particles at thebeginning of the dryer. The uniform field holds the particles in thepreferred direction as the solvent is removed in a uniform dryingenvironment to such a level that the viscosity is increased to the pointthat the viscous forces dominate. This prevents undesirable particledisorientation as it leaves the orienting device or from interparticleforces. (Drying at elevated rates in conventional dryers causes thesurface of the product to roughen.) Removing the solvent in thecontrolled environment of the dryer of this invention appears to createsmoother surfaces at elevated solvent removal rates. This also improvesmagnetic output as, for example, the resulting tape will ride closer tothe recording head.

Various changes and modifications can be made in the invention withoutdeparting from the scope or spirit of the invention. For example, theinvention has been described as a method and apparatus for performingmagnetic orientation on a web while the web is inside a dryer bylocating a magnetic field generator outside of the dryer. Otheroperations can also be performed on a web inside of a dryer from adevice located outside the dryer when the dryer is as compact as thedryer enabled by the present invention. For example, some operationsthat can be performed include curing such as by UV radiation or electronbeam radiation; decontaminating or sterilizing with, for example, aradiation source; and corona treating or light treating (whether throughthe dryer or through a hole or slot in the dryer plate through which thecorona or light can pass). Other operations that do not requirephysically contacting the web also can be performed from outside of thedryer. Also, magnetic orientation and other operations can be performedoutside of other types of dryers.

We claim:
 1. A method for drying a coated substrate, wherein the coated substrate comprises magnetic particles, comprising:locating a condensing surface spaced from and facing the substrate which substantially corresponds to the path of the substrate in the longitudinal direction to create a longitudinal gap between the substrate and the condensing surface; evaporating the liquid from the substrate to create a vapor; transporting the vapor to the condensing surface without requiring applied convection; condensing the vapor on the condensing surface to create a condensate; removing, using more than gravity, the condensate from the condensing surface without allowing non-uniformities of the condensate film to occur; and orienting the magnetic particles on the coated substrate by subjecting the coated substrate to a magnetic field between the condensing surface and the coated substrate that is initially created at a location outside of the space between the condensing surface and the coated substrate.
 2. The method of claim 1 wherein the orienting step comprises creating the magnetic field at least one of: a first location separated from the substrate by the condensing surface and a second location separated from the condensing surface by the substrate.
 3. The method of claim 2 wherein the orienting step comprises creating the magnetic field at locations surrounding the condensing surface and the substrate.
 4. The method of claim 1 wherein the orienting step comprises orienting the magnetic particles at the beginning of the evaporating step and holding the magnetic particles in a preferred direction during the evaporating, condensing, transporting, and removing steps.
 5. The method of claim 1 wherein the removing step comprises at least one of: tilting the condensing surface to at least one transverse side of the coated substrate such that gravity is used to remove the condensate from the condensing surface; and using mechanical shear forces.
 6. The method of claim 1 further comprising the step of providing relative movement between the condensing surface and the coated substrate.
 7. The method of claim 1 further comprising the step of controlling the rate of drying by controlling the height of the gap and the temperature difference between the coated substrate and the condensing surface.
 8. An apparatus for drying a coated substrate, wherein the coated substrate comprises magnetic particles, comprising:a condensing surface locatable spaced from and facing the substrate which substantially corresponds to the path of the substrate in the longitudinal direction to create a longitudinal gap between the substrate and the condensing surface; means for evaporating the liquid from the substrate to create a vapor; means for transporting the vapor to the condensing surface without requiring applied convection; means for condensing the vapor on the condensing surface to create a condensate; means for removing, using more than gravity, the condensate from the condensing surface without allowing non-uniformities of the condensate film to occur; and at least one magnetic field generator located outside of the space between the condensing surface and the coated substrate.
 9. The apparatus of claim 8 wherein the magnetic field generator is located at at least one of: a first location separated from the substrate by the condensing surface and a second location separated from the condensing surface by the substrate.
 10. The apparatus of claim 9 wherein the magnetic field generator is located at a location surrounding the substrate and the condensing surface such that the substrate passes through the magnetic field generator.
 11. The apparatus of claim 8 wherein the magnetic field generator is located at the entrance to the condensing plate.
 12. The apparatus of claim 8 wherein the evaporating means comprises a heated plate which supplies heat by conduction to increase the rate of heat transfer compared to conventional drying methods that use convection.
 13. The apparatus of claim 8 wherein the removing means comprises at least one of: tilting the condensing surface to at least one transverse side of the coated substrate such that surface tension holds the condensate onto the condensing surface and gravity is used to remove the condensate from the condensing surface; forming the condensing surface of a foraminous material; and a wiper.
 14. The apparatus of claim 8 wherein the condensing platen is sloped away from the horizontal in any direction to remove condensate by gravity.
 15. The apparatus of claim 8 further comprising a belt, wherein the condensing surface is formed on the belt and means for providing relative movement between the condensing surface on the belt and the coated substrate.
 16. The apparatus of claim 8 further comprising means for controlling the rate of drying by controlling the height of the gap and the temperature difference between the coated substrate and the condensing surface.
 17. The apparatus of claim 8 further comprising: a plurality of condensing surfaces; wherein the evaporating means comprises a plurality of heaters, and wherein each heater corresponds to a respective condensing surface to form pairs of condensing surfaces and heaters; and means for independently controlling each pair of condensing surface and heater.
 18. An apparatus for drying a coating on a substrate, wherein the coating comprises particles in a liquid, the particles being capable of being affected by a magnetic field, the apparatus comprising:a condensing surface locatable spaced from the substrate along the path of the substrate to create a gap between the substrate and the condensing surface; means for evaporating the liquid from the coating to create a vapor; means for condensing the vapor on the condensing surface to create a condensate; and a magnetic field generator located outside of the space between the condensing surface and the coated substrate for imposing a magnetic field upon the particles in the coating.
 19. The apparatus of claim 18 wherein the coated substrate is useful for creating at least one of an audio, video, or data storage medium, wherein the magnetic field generator is configured to magnetically align the particles.
 20. A method for drying a coating on a substrate, wherein the coating comprises particles in a liquid, the particles being capable of being affected by a magnetic field, the method comprising the steps of:locating a condensing surface spaced from the substrate along the path of the substrate to create a gap between the substrate and the condensing surface; evaporating the liquid from the coating to create a vapor; condensing the vapor on the condensing surface to create a condensate; and creating a magnetic field from outside of the space between the condensing surface and the coated substrate and imposing the field upon the particles in the coating.
 21. The method of claim 20 wherein the coated substrate is useful for creating at least one of an audio, video, or data storage medium, wherein the magnetic field aligns the particles.
 22. The method of claim 21 further comprising the step of converting the coated substrate into one of an audio, video, or data storage media product following the evaporating step. 